(1) Field of the Invention:
The present invention relates to metal-ceramic joined composite bodies and a joining process therefor. More particularly, the invention relates to metal-ceramic joined composite bodies in which a metallic member and a ceramic member are integrally joined together by brazing or a mechanical means such as press fitting as well as a joining process therefor.
(2) Related Art Statement:
Since ceramics such as zirconia, silicon nitride, and silicon carbide have excellent properties such as mechanical strength, heat resistance and wear resistance, they have been put into practical use as high temperature structural materials or wear-resisting materials for gas turbine engine parts, engine parts, etc. However, since ceramics are generally hard and brittle, and they are inferior in shapability and processability to metallic materials. Further, since ceramics have poor toughness, they have poor impact resistance. For these reasons, it is difficult to form mechanical parts such as engine parts from ceramic materials only. Thus, ceramics are generally used in the form of composite structures in which a metallic member is joined to a ceramic member.
For instance, Japanese Utility Model Registration Application Laid-open No. 61-108,329 discloses a structural body in which a columnar fitting portion of a ceramic member is fitted into a cylindrical fitting hole of a metallic member and both the members are integrally joined together by brazing. However, when members made of two kinds of materials having different coefficients of thermal expansion are joined together between the entire outer surface of the columnar fitting portion and the entire inner surface of the cylindrical fitting hole as in this structure, excess residual stress occurs so that the ceramic member is likely to break. Thus, there remains a problem in a practical use.
In order to solve the above problem, it has been attempted to limit a joining area between a ceramic member and a metallic member to a specific zone. For instance, Japanese patent application Laid-open No. 61-219,766 discloses a structure in which an end portion of a ceramic shaft is inserted into a blind hole formed in a metallic shaft, a tip end surface of the ceramic shaft and a bottom surface of the blind hole to be butted together are joined through an intermediate layer which can be bonded to both the ceramic and the metal, while between the outer peripheral surface of the ceramic shaft and the inner peripheral surface of the blind hole of the metallic shaft is interposed another intermediate material which is not bonded to either the ceramic or the metal.
Further, EP-A-0195640 discloses a process for joining a structural body in which a projection formed on a ceramic member is inserted into a recess formed in a metallic member, and the outer peripheral surface of the projection is integrally joined to the inner peripheral surface of the recess through an active brazing metal, while a space is provided between a tip end surface of the projection and a bottom surface of the recess.
Furthermore, Japanese patent application Laid-open No. 61-169,164 discloses a joint structure in which a projection formed on a ceramic member is fitted into a sleeve portion formed in a metallic member, and the outer peripheral surface of the projection is partially bonded to the inner peripheral surface of the sleeve portion with a brazing metal.
However, according to the structure described in Japanese Patent Application Laid-open No. 61-219,766, if the interference fitting effect is lost between the outer peripheral surface of the ceramic shaft and the inner peripheral surface of the blind hole of the metallic shaft due to rises in use temperatures, necessary strength must be assured by a joined portion between the butted tip end surface of the ceramic shaft and the bottom surface of the blind hole of the metallic shaft. Thus, if the ceramic shaft is slender and the use temperature is high, it is unfavorably difficult to assure such necessary strength.
On the other hand, according to the process for joining the structure disclosed in No. EP-A-0195640, an active brazing metal is arranged between the surface of the ceramic member and the metallic member to be joined together, and both the members are joined together by melting the brazing metal. When the brazing is carried out in this manner, defects such as bubbles or shrinkage cavities are likely to occur in the brazing metal so that sufficient joining strength cannot be obtained.
According to the structure described in Japanese Patent Application Laid-open No. 61-169,164, a metallizing layer is formed on a joining surface of the projection formed on the ceramic member, and the metallizing layer is brazed to the inner peripheral surface of the metallic member. Although this method is excellent for the purpose of limiting a portion of the ceramic member to be brazed, it not only needs an extra step for metallizing the ceramic member, but also the ceramic member must be made of a ceramic material which can allow the formation of the metallizing layer.
The above problems are to be first solved by the present invention.
Problems of the following prior art have been considered to be secondarily solved by the present invention.
Japanese Utility Model Registration Application Laid-open No. 59-5,701 discloses a structure in which a sleeve made of a metallic material having a coefficient of thermal expansion near that of a ceramic vane wheel is connected to a boss of the ceramic vane wheel, and the metallic sleeve is connected to a metallic shaft, while a space is provided between the ceramic vane wheel and the metallic sleeve.
Japanese Patent Application Laid-open No. 62-78,172 discloses a structure in which a ceramic shaft member is joined to a metallic sleeve member by fitting, and a groove is formed around the outer periphery of the sleeve along a position corresponding to a joining end between the sleeve and the ceramic shaft member.
In the method for joining the structure disclosed in Japanese Patent Application Laid-open No. 62-78,172, a brazing metal is arranged between surfaces of the ceramic shaft and the sleeve to be joined together, and the ceramic shaft and the metallic shaft are integrally joined together with the buffer metallic member by melting the brazing metal in situ. Thus, defects such as bubbles or shrinkage cavities are likely to occur in the brazing metal, so that sufficient joining strength cannot be obtained.
In the structure disclosed in Japanese Utility Model Registration Application Laid-open No. 59-5,701, since stress is likely to concentrate upon the joining end between the boss of the ceramic vane wheel and the metallic sleeve during use, joining strength is unfavorably poor.
Such problems are to be secondarily solved by the present invention.
Problems of the following prior art are to be thirdly and fourthly solved by the present invention.
Metal-ceramic composite bodies have been known, which are designed such that in a joined portion in which an outer surface of a projection of a ceramic member is joined to an inner surface of a recess of a metallic member by brazing, the projection is firmly fixed to a brazing metal present between the outer surfaces of the projection of the ceramic member and the inner surface of the recess of the metallic member through chemical joining over the entire surface thereof.
In general, when a projection of a ceramic member is to be joined to a recess of a metallic member by brazing, assume a case where temperature is lowered from a solidifying point of the brazing metal to room temperature. In this case, since the metallic member or the brazing metal has a greater coefficient of thermal expansion while that of the ceramic member is smaller, shrank amounts of the metallic member or the brazing metal are greater due to difference in coefficient of thermal expansion. However, as mentioned above, in the structure in which the projection of the ceramic member is firmly fixed to the brazing metal over the entire contact surface between the projection and the brazing metal through chemical joining, the brazing metal is firmly fixed to the ceramic member due to shrinkage of the metallic member or the brazing metal during cooling. Consequently, the brazing metal cannot slip relative to the ceramic member at their joining interface so that the shrinking force of the metallic member or the brazing metal acts upon the ceramic member and excessive tensile stress occurs in the joining end of the ceramic member. Further, no investigations have been considered upon the relation between the surface of the tip end of the projection and the bottom surface of the recess.
Therefore, tensile stress concentration particularly upon the joining end of the ceramic member increases, which reduces resistance of the joined composite body against bending or twisting, so that reliability is deteriorated.
Such problems are to be thirdly fourthly solved by the present invention.
The following problems are to be fifthly solved by the present invention.
Furthermore, Japanese Utility Model Registration Application Laid-open No. 61-164,234 discloses a structure in which a projection of a ceramic member is joined to a recess of a metallic member with a brazing metal, and a space is formed on an end side of an opening of the recess to mitigate stress concentration occurring at the joining end due to difference in thermal expansion between the ceramic member and the metallic member or between the ceramic member and the brazing metal, while the members are joined at the remaining portion with the brazing metal. Japanese Utility Model Registration Application Laid-open No. 60-180,137 proposes that the brazing metal is interposed up to a radially outwardly enlarged portion formed at an opening portion of a recess of the metallic member in such a structure.
Although stress concentration upon the joining end can be mitigated to some extent by the joining structures in the above-mentioned prior art, satisfactory effect can not be obtained. That is, since a sufficient space is not provided at a joining bottom portion between the surface of the tip end of the projection of the ceramic member and the bottom surface of the recess of the metallic member, the ceramic member and the metallic member are brought into contact with each other due to a difference in thermal expansion therebetween during cooling from the solidifying point of the brazing metal to room temperature on brazing. Since the ceramic member has a smaller thermal expansion than that of the metallic member, the metallic member cannot shrink sufficiently to cause too much tensile stress upon the ceramic member. Since the metallic member cannot be fully shrunk due to the ceramic member having a smaller thermal expansion than that of metallic member, excess tensile stress occurs in the ceramic member. Since stress concentration particularly upon the joining end increases, resistance of the joined composite body against bending or twisting is lowered.
The problems mentioned just above are to be fifthly solved by the present invention.